This invention relates generally to circuit board testers and, in particular, to a circuit board tester having a non-contact, electrically conductive grounding or reference plate.
In the art of packaging and interconnecting electronic devices, there is a notable trend towards devices of increased complexity and density. Regarding circuit boards in particular, there is a definite gravitation towards multiple layer circuit boards having large numbers of closely spaced circuit runs. The presence of internally arranged conductors in these boards makes visual inspection of the conductors for electrical continuity and electrical shorts virtually impossible, so that testing techniques must be employed which are based upon the electrical conductivity of the conductors.
In the art of circuit board testers, devices are known for testing the electrical integrity of circuit runs by contacting the ends of the circuit runs, either in step-and-repeat fashion using two probes or in simultaneous fashion using a fixed probe tester, and observing the electrical characteristics.
When a two-probe tester is used, one probe is placed in contact with one end of the circuit run to be tested. The other end or ends of this particular run are then contacted by the other probe to test for electrical continuity. In order to test for shorts, the second probe is sequentially applied to all of the other circuit runs to determine whether a closed circuit condition is indicated, representing the fact that the circuit run under test is shorted to one of the other circuit runs. For this type of testing, at least N + (N-1) + (N-2) + . . . +1 tests are required, N representing the number of circuit runs for a given circuit board.
With a fixed probe tester, a probe must be provided for each end of all circuit runs, and the number of probes required may be in the order of several thousand. To check a particular circuit run for electrical continuity using a fixed probe tester, all of the probes are electrically connected to the tester. A good electrical path will indicate a low resistance reading between the probes associated with the ends of the circuit run under test. To test for shorts, all runs except the one to be tested are grounded to one side of the tester, and a probe connected to the other side of the tester is applied to the circuit run under test. The circuit run under test should provide a high resistance reading if it is not shorted to any of the other circuit runs.
Fixed probe type testers are very expensive due to their complexity and to the relatively high maintenance costs associated with their fragile, short-lived probe heads. Testers of the two-probe type are relatively low in cost compared to the multiple-probe type; however, the test cycle is extremely time-consuming.
A further known circuit board tester overcomes certain of the disadvantages of the other prior art testers, in that it requires only two probes and only one measurement for each circuit run tested. This tester, described in "Continuity Testing by Capacitance," Robert W. Wedwick, Published Circuits Manufacture, Nov., 1974, pp. 60 and 61, utilizes measurements of the distributed capacitance between a circuit run and a reference plane such as a ground or power plane to identify electrical shorts and discontinuities. By comparing the measured capacitance value for a particular circuit run with the capacitance of a known good circuit run, an accurate indication of the electrical integrity of the measured circuit run can be obtained. Using this type of tester, one of the test probes is attached to the ground or power plane of the circuit board, and the other test probe is sequentially applied to the end of each circuit run, taking a capacitance reading for each circuit run and comparing such reading to a previously obtained capacitance value for a known good circuit run.
The present invention is directed to an improvement over the last-mentioned type of circuit board tester, in that it eliminates the necessity for directly connecting the ground or reference side of the tester to the ground plane or power plane of the circuit board, thereby substantially increasing the operational speed of the testing process and simplifying the design of the tester.